There have been proposed electricity supply systems that supply electricity to loads (appliances such as a refrigerator, a washing machine, and an air conditioner) by using not only electricity supplied (purchased) from an electricity distribution facility (hereinafter, an electricity system) managed by an electricity company (such electricity will hereinafter be referred to as “system electricity”) but also various kinds of electricity (various electricities) generated by various methods, such as electricity that is generated by, and supplied from, a power generating unit such as a solar photovoltaic power generating unit, and electricity discharged and supplied from a battery. Some of such electricity supply systems supply (sell), to the electricity system, excess electricity that is generated and supplied by an electricity generator but is not consumed by a load or the like.
Electricity consumption of a load may include general electricity consumption (caused by, for example, the operation of an appliance such as a refrigerator, for example) which constantly occurs regardless of time of day, and particular electricity consumption (caused by, for example, the operation of an appliance such as a washing machine) which occurs exclusively at a particular time of day. Where reduction of system electricity cost is required, it is effective to reduce cost of the particular electricity consumption. For example, it is possible to reduce the particular electricity consumption cost by setting the particular electricity consumption to occur at a time when per-unit price of the system electricity (hereinafter, an electricity purchase rate) is low.
For example, Patent Literature 1 proposes an electricity supply system that informs a user of time when the particular electricity consumption can be covered with excess electricity (and thus the particular electricity consumption cost can be reduced to 0), to thereby prompt the user to set the particular electricity consumption to occur at that time. Furthermore, this electricity supply system displays the electricity purchase rate and a per-unit selling price of electricity (hereinafter, an electricity sales rate) to the user, to thereby assist the user in deciding whether to sell the excess electricity or use it to cover the particular electricity consumption therewith.